Amplifier is configured to convert a low power signal to a high power signal, and it is desired to have high quality and low total harmonic distortion (THD).
Generally, an amplifier is powered by a positive power supply and a system ground (zero voltage). The output voltage of the amplifier is only allowed to oscillate between the zero voltage and the positive power supply voltage. It turns out that the output end of the amplifier could not be biased to the system ground when the load of the amplifier is coupled to the system ground. As a result, a large static current exists in the path from the output end of the amplifier to the system ground. This static current may lower the efficiency of the amplifier and short the lifetime of the load.
In order to bias the output end of the amplifier to the system ground, in the amplifier circuit, a negative charge pump may be utilized and configured to generate a negative voltage (taking the system ground as zero reference) as the lowest power supply voltage instead of the system ground. The output voltage of the amplifier is oscillated between the positive power supply voltage and this negative voltage so that it is easy to bias the amplifier to the system ground. Unfortunately, the negative charge pump enlarges the voltage drop on the amplifier. Even in low power occasion, it is difficult to fabricate this type of amplifier through a low voltage low cost semiconductor process. For example, if the amplifier is utilized at cell phone charger, since the maximum voltage of the Li-on battery is 4.2V, the voltage drop on the amplifier may be larger than 8V. The amplifier with such large voltage drop may be difficult to fabricate. Even being fabricated, it may have a relative high cost.
For reducing the voltage drop on the amplifier, low dropout regulator (LDO) may apply in the amplifier circuit. The LDO receives the positive power supply voltage and provides a relative low positive voltage to the amplifier. Therefore the voltage drop becomes lower and the amplifier is easy to fabricate.
However, for conventional charge pump, its output voltage may tend to be affected by the input voltage. When the input voltage changes, the output voltage changes correspondingly which makes the output of the amplifier unstable and the PSRR worse.